1. Field of the Invention
The present invention relates to camera photographic systems for flash photography, and particularly to devices capable of controlling the amount of light emitted by electronic flash firing in such photographic systems.
2. Description of the Prior Art
Prior art electronic flash devices in flash photographic systems of the controllable flash light output type change the flash duration to control the amount of flash light, but the operating mode of the flash tube remains unchanged. In such cases, however, a controlled decrease in the light output inevitably results in excessive light output. Thus, this type of prior art electronic flash device poses a problem of control when a small amount of flash light is required for certain photographic modes, such as closeup photography.
Prior art electronic flash devices of the controllable light output type are generally constructed as shown in FIG. 1. DC-DC converter 1 boosts the voltage of power cell 2, and the output of DC-DC converter 1 is rectified by diode 4 to charge main capacitor 5. A trigger circuit (not shown) triggers flash tube 7, and at the same time, turns thyristor 6 on, whereby flash tube 7 emits flash light having a waveform shown by curve l in FIG. 2. When switch S1 is turned on at point t1 of the flash duration, the potential at node A becomes zero. As, however, commutation capacitor C is charged with a positive voltage, the potential at node A being zero causes the potential at node B to decrease to a negative voltage, which in turn makes the anode of thyristor 6 at a negative potential, whereby thyristor 6 is cut off to stop the flash firing of flash tube 7. However, the flash firing does not stop immediately, as node B remains at a negative potential because of capacitor C, and as the discharge current of flash tube 7 flows into capacitor C, the voltage at node B increases, whereby flash tube 7 continues to be discharged for flash firing until the voltage across both terminals of flash tube 7 reaches a level at which the discharge can continue no longer. The spike-like waveform of flash light from points t1 to t2 above the dotted line in FIG. 2 is for flash light that continues after point t1, and is called the excessive amount of light, as described above. The excessive amount of light exists even when switch S1 is closed at point t3 in FIG. 2. In this case, however, the ratio of the excessive light output to the necessary light output is small. However, in the case where a small amount of light is required for closeup photography or the like, the ratio of an excessive amount of flash light to a desired amount of flash light becomes large, resulting in a considerable deleterious effect that is not ignorable when a small amount of flash light is required.